The present invention generally relates to digital communication over radio frequency (RF) channels and, more particularly, to providing bandwidth efficient modulation of RF signals for digital communications through satellite transponders.
At present, commercial satellite transmission equipment for digital communications is generally limited to quadrature phase shift keying (QPSK) or eight phase shift keying (8-PSK) modulation that limits data rates to approximately 30 to 45 megabits per second (Mbps) on a standard satellite transponder channel. For commercial users wishing to transfer large data files, it can take approximately eight hours for transmission, for example, of a sixty giga-byte data file at data rates between 30 Mbps and 45 Mbps. The use of a satellite transponder channel for such a length of time can represent a significant amount of expense for such a commercial user. Thus, any reduction in transmission time that can be achieved through more efficient use of the satellite transponder communication channel can provide significant cost savings, and advantages of speed and increased channel capacity. The same considerations may be applied not only to the transfer of large data files but, more generally, to increased bandwidth efficiency for all types of satellite communications, including telecommunications.
The prior art for digital file transmission over satellite transponder communication channels is overwhelmingly dominated by commercial QPSK modulators and receivers because their constant-envelope modulation is not adversely affected by the nonlinear distortion introduced at the satellite transponder. Typically, nonlinear distortion dictates that satellite communication applications use the lower-order constant-envelope modulation types such as QPSK and 8PSK. This is because the need for satellite power efficiency requires operating transponders near saturation where the input/output curve is flattened out so that the amplitude variations going into the amplifier are not being represented properly at the output, i.e., amplitude is distorted. Thus, amplitude distortions can prevent the use of higher order modulation types, such as quadrature amplitude modulation (QAM) that depend on non-distorted amplitude transmission. Traveling-wave tubes (TWTs) are still used for much of the satellite communications application, especially in geosynchronous communications satellites. TWTs typically demonstrate nonlinear distortion that is amplitude dependent and memoryless.
Amplifier nonlinearity is present, as well, in other types of communications systems. Cell phone networks, for example, in which a mobile station (the cell phone) communicates with a base station using RF signals, may be subject to analogous limitations and problems to those of the satellite transponder channel. Also, for example, code division multiple access (CDMA) systems are known to be sensitive to nonlinearity. If the nonlinearity could be compensated for, improvements in performance and efficiency could be expected for a number of types of communications systems, including cell phone and CDMA communications systems
Predistortion is an active technology in all wireless communications applications. Typically, a predistorter is installed on the front end of a satellite transponder to reduce intermodulation products and adjacent channel interference. Predistorters are often implemented as an analog gain (amplitude) decompressor and phase shifter that compensates for the gain compression and phase shift within the power amplifier. Predistorters are typically inserted in the communications channel somewhere after the modulator and the RF upconverter, but before the power amplifier. An analog predistorter for satellite communications may be implemented on the ground, for example, by predistorting the modulated RF signal (to match the particular satellite transponder) at the ground transmitter before it is inserted into the power amplifier and then transmitted. Analog predistorters typically attempt to provide a continuous predistortion over the whole range of amplifier input power. Thus, an analog predistorter must be designed specifically to match the characteristics, for example, transfer function, of each model of each particular manufacturer's line or class of transponder amplifiers. For example, an inverted gain that compensates for the amplitude distortions of the transponder needs to be applied to the modulated signal before transmission, but it can be extremely difficult to properly match the required gain compensation across all power levels of transmission. The custom-tailored nature of differently matching each class of amplifiers can be expensive and can hamper availability.
As can be seen, there is a need for a predistortion modulator to enhance bandwidth efficiency of digitally modulated signals for communications systems, including satellite communications systems. There is also a need for a predistortion modulator for transmission of digitally modulated signals that easily may be used with a wide variety of satellite transponders and amplifier classes.